Dehydrogenation catalyst and process for preparing same

ABSTRACT

A calcined dehydrogenation catalyst comprising at least one sodium compound and one calcium compound is disclosed. The catalyst exhibits improved moisture stability as evidenced by improved crush strength and resistance to swelling and cracking.

BACKGROUND OF THE INVENTION

This invention relates to improved catalysts for the dehydrogenation ofhydrocarbons and to a method of making such catalyst compositions whichexhibit improved moisture stability when subjected to contact withhydrogen and steam at elevated temperatures during the dehydrogenationprocess. This improved stability is evidenced by improved crush strengthand resistance to swelling and cracking.

TECHNICAL BACKGROUND

Catalytic dehydrogenation of hydrocarbons using various catalystcompositions has been known from just prior to World War II. Commercialexamples are the manufacture of styrene and butadiene from ethylbenzeneand butylene. Promoted iron oxide catalysts have been found to beespecially useful in the dehydrogenation of alkyl aromatic hydrocarbonsto vinyl aromatic hydrocarbons. Most commercial iron oxidedehydrogenation catalysts include minor amounts of promoters, e.g.,salts or oxides of chromium, manganese, bismuth, tungsten, ormolybdenum, with chromium being preferred, together with a compound ofpotassium, e.g., potassium oxide or carbonate. The potassium compoundgives the catalyst a self-regenerative property that prolongs its usefullife for long periods of time without significant loss of activity.Recent improvements include the incorporation of minor amounts ofvanadium and modifiers, such as carbon black or graphite and methylcellulose, which can beneficially affect the pore structures of thecatalysts.

The catalyst life of dehydrogenation catalysts is often dictated by thepressure drop across a reactor. An increase in the pressure drop lowersboth the yield and conversion to the desired product. Physicaldegradation of the catalyst typically increases the pressure drop acrossthe reactor. For this reason, the physical integrity of the catalyst isof major importance. Dehydrogenation catalysts containing iron oxide canundergo substantial changes under process conditions which decreasetheir physical integrity. For example, in the dehydrogenation ofethylbenzene to styrene, the catalyst is subjected to contact withhydrogen and steam at high temperatures and, under these conditions, Fe₂O₃, the preferred source of iron for the production of styrenecatalysts, is reduced to Fe₃ O₄. This reduction causes a transformationin the lattice structure of the iron oxide, resulting in catalyst bodieswhich have poorer physical integrity and are very susceptible todegradation by contact with water at temperatures below 100° C. Thisdegradation by contact with water is characterized by the catalystbodies (e.g., pellets or granules) becoming soft and/or swollen and/orcracked. The water which contacts the catalysts may be in the form ofliquid or a wet gas, such as air with a high humidity. "High humidity"refers to a relative humidity above about 50%.

The catalysts in styrene production plants are often exposed totemperatures below 100° C. during start-ups, shutdowns, and upsets.Because large amounts of steam are used in styrene production, there issignificant potential for exposing the catalyst to moisture at lowtemperatures.

As previously discussed, this exposure causes physical degradation ofthe catalyst, which increases pressure drop across the reactor,resulting in decreased catalyst life.

In recent years, catalysts with higher amounts of potassium have beenused: in U.S. Pat. No. 4,503,163 assigned to Mobil Oil Company, forexample, catalysts are disclosed which contain 13-48% and preferably27-41% by weight of a potassium promoter compound, calculated aspotassium oxide. Such catalysts are self regenerative catalysts whichperform well at lower steam to oil ratios; e.g., ratios of<2:1 (byweight). The economic advantages of using less steam are obvious. Theproblem with using higher concentrations of potassium is that thevulnerability of the used iron oxide catalyst to moisture increases withincreasing potassium concentration.

A need exists for a dehydrogenation catalyst that has both highactivity, selectivity and resistance to moisture. A way has now beendiscovered to enhance moisture resistance of these catalysts without anysignificant detrimental effects to catalyst performance.

SUMMARY OF THE INVENTION

In one embodiment the invention is a novel calcined dehydrogenationcatalyst comprising (a) an iron oxide; (b) a potassium and/or cesiumcompound; (c) at least one sodium compound in an amount from about 0.2to about 10 weight percent sodium, calculated as sodium oxide; and (d)at least one calcium compound in an amount from 1.5 to 20 weight percentcalcium calculated as calcium oxide.

In a second embodiment, the invention is a process for preparing suchimproved dehydrogenation catalysts. This process comprises the steps of(a) preparing an extrudate by admixing iron oxide, a potassium and/orcesium compound, at least one sodium compound in an amount from about0.2 to about 10 weight percent sodium calculated as the oxide, at leastone calcium compound in an amount from 1.5 to 20 weight percent calciumcalculated as calcium oxide and sufficient water to form an extrudablemixture; (b) forming said extrudable mixture into pellets; and (c)calcining said pellets into a finished catalyst.

The new catalyst compositions are useful for the dehydrogenation of analkyl aromatic compound to form a vinyl aromatic compound by contactingthe alkyl aromatic compound with the dehydrogenation catalyst underdehydrogenating conditions. The new catalysts have improved moisturestability, as evidenced by improved crush strength, which decreasescatalyst degradation during process upsets.

DETAILED DESCRIPTION OF THE INVENTION

The invention resides in the discovery that the addition of sodium andcalcium compounds to known dehydrogenation catalysts produce newdehydrogenation catalysts having improved stability. Thus, any of theknown class of dehydrogenation catalyst compositions containing red oryellow iron oxides and various catalyst promoters (as disclosed, forexample, in U.S. Pat. Nos. 4,503,163, 3,703,593, 4,684,619, all of whichare assigned to The Dow Chemical Company, and are incorporated byreference) may be used herein. Iron is generally added to the catalystcompositions of the invention as red iron oxide, Fe₂ O₃, or yellow ironoxide, Fe₂ O₃.H₂ O. Particularly suited are pigment grades of red andyellow iron oxides. Likewise the catalyst promoter can be any materialtaught by the art, for example, an alkali metal compound(s) that isconverted to an alkali metal oxide under calcination conditions.Potassium compounds are the preferred promoters. The promoter can beadded to the catalyst in various forms. The alkali metal oxides,hydroxides, carbonates, bicarbonates, and the like, and mixtures thereofare preferred, and potassium carbonate or a mixture of potassiumcarbonate with potassium oxide is most preferred.

The catalyst compositions of the present invention also may contain, andpreferably do contain, cerium to enhance selectivity. Cerium, if used inthe catalyst compositions of the present invention, can be added to thecatalyst in the form of cerium oxide or in the form of other ceriumcompounds that decompose upon calcination to form cerium oxide, as forexample, cerium carbonate, cerium nitrate, cerium hydroxide or anycombination thereof.

Other known catalyst additives can be included in the catalysts of theinvention, but are not essential. A chromium compound which can serve asa stabilizer for the active catalytic components is illustrative of anoptional but preferred additive. Chromium compounds have previously beenadded to alkali-promoted iron oxide catalysts to extend their life.Chromium, as used in the compositions of this invention, can be added tothe catalyst in the form of a chromium oxide or in the form of chromiumcompounds which decompose upon calcination to chromium oxides.

Another optional component, used to improve the selectivity of thecatalyst, is molybdenum which can be added as its oxide or as amolybdate. Other metal compounds that may be added as promoters includecompounds of aluminum, vanadium, cobalt, cadmium, copper, magnesium,manganese, and nickel, providing they can be calcined to thecorresponding metal oxide.

The physical strength, activity and selectivity of the catalystcompositions of the present invention can be improved by adding certainbinding agents. Binding agents can include and consist of hydrauliccements, for example, calcium aluminate or Portland cement. Thesecements can be added individually or in combination.

The density of the catalyst composition can be modified by the additionof various filler substances, for example, combustible materials such asgraphite and methyl cellulose. Such materials can be added to thecompositions during preparation, but are burned out after the catalystpellets have been formed during the calcining step. Porosity promotingaids can also facilitate extrusion of catalyst pellets.

The catalyst components can be mixed in various ways known to the art.One method comprises ballmilling together a mixture of desiredcompounds, adding a small amount of water, and extruding the compositeto produce small pellets, which are then dried and calcined. Anothermethod is mixing the components together with water, drying them to forma powder and tabletizing and calcining the tablets. Another procedureinvolves mixing the components together with an excess of water,partially drying, and then subsequently extruding, drying, and calciningthe resulting pellets. The choice of the mixing method depends on thepreference of the skilled artisan.

A preferred method of preparing the catalysts is to blend the catalystingredients together, including the ingredients of the presentinvention, in the presence of sufficient water to make a moistextrudable mixture. This mixture is then extruded to produce extrudatesbetween 1/8-inch and 1/4-inch in diameter. The extrudates are thencalcined under conventional calcining conditions. Calcinationtemperatures can range from about 500° C. to about 900° C., preferablyfrom about 600° C. to about 800° C. After calcination the extrudates areready for use as catalysts.

The amount of sodium included in the new catalyst, measured as sodiumoxide and based on the weight of the calcined catalyst may range fromabout 0.2% to about 10%. Preferably sodium is present in the catalyst inthe range of about 0.5% to 5%. Most preferably, sodium is present inamounts of from 0.8 to 3.0%. The sodium may be added to the catalystmixture as sodium hydroxide or carbonate or bicarbonate or other saltssuch as acetate, oxalate, nitrate, or sulfate.

The amount of calcium in the new catalyst, measured as calcium oxide andbased on the weight of the calcined catalyst, may range from 1.5% to20%. Preferably calcium is present in the range of about 3% to 15%. Mostpreferably calcium is present in amounts from 4% to 12%. Calcium can beadded to the catalyst mixture in the form of calcium carbonate, calciumsulfate, calcium hydroxide, or other salts.

EXPERIMENTAL

All catalysts prepared in the following examples are prepared fromcommercially available chemicals. LUMNITE is the trade name for calciumaluminate cement manufactured by Lehigh Cement Company.

Examples 1-5

A catalyst formulation is made by blending in a heated steam jacketedblade mixer 135 grams(g) red iron oxide (Fe₂ O₃), 153.4 g yellow ironoxide (Fe₂ O₃.H2O), 120 g calcium aluminate cement (LUMNITE), 38 ggypsum (CaSO₄.2H₂ O), 120 g calcium carbonate (CaCO₃), 10 g molybdenumoxide (MoO₃), 190.5 g hydrated cerium carbonate (Ce₂ (CO₃)₃.5H₂ O), 450g potassium carbonate (K₂ CO₃), 10 g potassium dichromate (K₂ Cr₂ O₇),and 41.5 g of a aqueous 50% solution of sodium hydroxide (NaOH). About15% (wt) of water is then blended into the formulation. The mixture ismixed and dried in the heated steam jacketed mixer until the formulationreaches a consistency suitable for extrusion. The hot catalyst mixtureis transferred to a California Model CL-3 Laboratory Pellet Mill andextruded (5/32" in diameter and about 10/32" in length).

Four additional catalysts formulations are prepared according to theabove procedure, but with different amounts of ingredients. Table 1lists the formulations of Examples 1-5. The extrudates of Examples 1-5are then calcined by slowly ramping the temperature to 775° C. over aperiod of about two hours, and maintaining at 775° C. for 30 minutes.The finished compositions of the calcined catalysts, expressed as shown,are given in Table 2.

                  TABLE 1                                                         ______________________________________                                        Catalyst Formulation                                                                    Ex.      Ex.     Ex.    Ex.   Ex.                                   Component*                                                                              1        2       3      4     5                                     ______________________________________                                        Fe.sub.2 O.sub.3                                                                        135      135     135    135   135                                   Fe.sub.2 O.sub.3.H.sub.2 O                                                                153.4    153.4   153.4                                                                                153.4                                                                               153.4                               LUMNITE   120      120     120    120   120                                   MoO.sub.3  10       10      10     10    10                                   Ce.sub.2 (CO.sub.3).sub.3.5H.sub.2 O                                                      190.5    190.5   190.5                                                                                190.5                                                                               190.5                               K.sub.2 CO.sub.3                                                                        450      450     450    450   450                                   K.sub.2 Cr.sub.2 O.sub.7                                                                 10       10      10     10    10                                   CaSO.sub.4.2H.sub.2 O                                                                    38       38      38     38    38                                   CaCO.sub.3                                                                              120      240      60     60    60                                   NaOH, 50%   41.5     45.8    19.5   39.3                                                                                66.2                                ______________________________________                                         *All weights in grams.                                                   

                  TABLE 2                                                         ______________________________________                                        Components of Calcined Catalyst                                                        Ex.     Ex.      Ex.    Ex.    Ex.                                   Component*                                                                             1       2        3      4      5                                     ______________________________________                                        Fe.sub.2 O.sub.3                                                                       23.46   21.21    25.0   24.78  24.48                                 LUMNITE  10.43   9.43     11.11  11.01  10.88                                 MoO.sub.3                                                                               0.87   0.79     0.93   0.92   0.91                                  Ce.sub.2 O.sub.3                                                                       10.43   9.43     11.11  11.01  10.88                                 K.sub.2 CO.sub.3                                                                       39.10   35.35    41.68  41.30  40.79                                 K.sub.2 Cr.sub.2 O.sub.7                                                                0.87   0.79     0.93   0.92   0.91                                  CaSO.sub.4                                                                              2.61   2.36     2.78   2.75   2.72                                  CaCO.sub.3                                                                             10.43   18.85    5.56   5.51   5.44                                  NaOH      1.80   1.80     0.90   1.80   3.00                                  ______________________________________                                         *Weight percent                                                          

COMPARATIVE EXPERIMENTS 1to 4

The same procedure employed in Examples 1-5 was used to make all thecomparative catalysts. Table 3 lists the formulations of the comparativecatalysts.

                  TABLE 3                                                         ______________________________________                                        Comparative Catalyst Formulation                                                          Comp.    Comp.    Comp.  Comp.                                    Component*  Ex. 1    Ex. 2    Ex. 3  Ex. 4                                    ______________________________________                                        Fe.sub.2 O.sub.3                                                                          135      135      135    135                                      Fe.sub.2 O.sub.3.H.sub.2 O                                                                  153.4    153.4    153.4                                                                                153.4                                  LUMNITE     120      120      120    120                                      MoO.sub.3    10       10       10     10                                      Ce.sub.2 (CO.sub.3).sub.3.5H.sub.2 O                                                        190.5    190.5    190.5                                                                                190.5                                  K.sub.2 CO.sub.3                                                                          450      450      450    450                                      K.sub.2 Cr.sub.2 O.sub.7                                                                   10       10       10     10                                      CaSO.sub.4.2H.sub.2 O                                                                      38       38      --      38                                      CaCO.sub.3  --        60      --     --                                       NaOH, 50%   --       --        36      39.3                                   ______________________________________                                         All weights in grams.                                                    

The finished compositions of the calcined comparative catalysts,expressed as shown, are given in Table 4.

                  TABLE 4                                                         ______________________________________                                        Components of Calcined Comparative Catalysts                                              Comp.    Comp.    Comp.  Comp.                                    Component*  Ex. 1    Ex. 2    Ex. 3  Ex. 4                                    ______________________________________                                        Fe.sub.2 O.sub.3                                                                          26.74    25.24    27.06  26.22                                    LUMNITE     11.88    11.21    12.02  11.65                                    MoO.sub.3    0.99     0.93     1.00   0.97                                    Ce.sub.2 O.sub.3                                                                          11.88    11.22    12.03  11.66                                    K.sub.2 CO.sub.3                                                                          44.55    42.05    45.09  43.70                                    K.sub.2 Cr.sub.2 O.sub.7                                                                   0.99     0.93     1.00   0.97                                    CaSO.sub.4   2.97     2.81    --      2.92                                    CaCO.sub.3  --        5.62    --     --                                       NaOH        --       --        1.80   1.91                                    ______________________________________                                         *Weight percent                                                          

The catalysts of the invention and those prepared for comparison aretested for activity and selectivity in the reaction for dehydrogenatingethylbenzene to styrene by placing 70 or 100 milliliters (mL) of theabove calcined catalyst extrudates in a fixed bed reactor and passing apreheated mixture of steam and ethylbenzene at a weight ratio of 1.5:1(called the steam to oil ratio) through the bed which is maintained at atemperature of 580°-590° C. The LHSV (liquid hourly space velocity) is1.0 and the pressure is maintained at atmospheric. The liquid hourlyspace velocity is a number denoting residence time in a reactor commonlyused by those skilled in the art. After a minimum of 5 days, the weightratio of steam to ethylbenzene is reduced to 1.2 and the bed temperatureadjusted so that an ethylbenzene conversion of 50% is achieved. Thistemperature adjustment is continued each day until a constant conversionof about 50% is achieved at a fixed bed temperature, that temperaturebeing an indication of the activity of the particular catalyst; i.e.,the lower the temperature, the higher the activity. The results of thedehydrogenation reaction for Examples 1-5 and Comparative Experiments1-4 are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                        Catalyst   Temp, °C.                                                                          Conversion                                                                              Selectivity                                  ______________________________________                                        Example 1  589         50.0      97.2                                         Example 2  597         49.8      97.1                                         Example 3  589         49.7      96.9                                         Example 4  596         50.3      96.5                                         Example 5  600         50.5      97.2                                         Comp. Ex. 1                                                                              598         50.2      95.9                                         Comp. Ex. 2                                                                              595         50.4      96.6                                         Comp. Ex. 3                                                                              590         50.4      97.1                                         Comp. Ex. 4                                                                              587         49.7      96.8                                         ______________________________________                                    

The moisture resistance of the used catalyst is measured by thefollowing method. After approximately two to three weeks of operation,the catalyst is unloaded from the reactor and twenty randomly chosenextrudates are placed in a glass dish with a fiat bottom. The extrudatesare separated so that they do not touch each other. The glass dish isthen placed in a controlled relative humidity chamber (Vapor-Temp ModelNo. VP-100AT-1) made by Blue M, a unit of General Signal, adjusted to30° C. and 70% relative humidity. After 20 hours the glass dish isremoved from the humidity chamber. The excess water in the dish isremoved, and the extrudates are placed in a drying oven at 150° C. for24 hours. The extrudates are removed and the average crush strength ofthe extrudates determined.

Crush strength is a common physical measurement that indicates thestrength of the catalyst body; i.e., tablet, pellet, or extrudate. Inthis test, catalyst bodies are compressed between two fiat metalsurfaces or blocks, and the pressure required to crush the body ismeasured. A Comten crush strength machine, model no. 922T-10-OP, serialno. 830202, is used according to the following procedure. The length ofthe extrudate is first measured, then the extrudate is crushed betweenthe two blocks of the Comten unit and the pressure required to crush theextrudate is recorded. The crush strength per one-quarter inch length ofextrudate is then calculated. This procedure is done on twenty randomlychosen extrudates from each catalyst sample which are all preconditionedas noted in the preceding paragraph. Averaged crush strength isexpressed as "PSIG/1/4 inch". The average of these twenty measurementsis shown in Table 6.

                  TABLE 6                                                         ______________________________________                                                 Average Crush                                                                 Strength                                                             Catalyst PSIG/1/4 inch.                                                                            Physical Appearance                                      ______________________________________                                        Example 1                                                                              18.5        Hard, No Cracking or Swelling                            Example 2                                                                              18.0        Hard, No Cracking or Swelling                            Example 3                                                                              13.5        Hard, No Cracking or Swelling                            Example 4                                                                              15.5        Hard, No Cracking or Swelling                            Example 5                                                                              18.8        Hard, No Cracking or Swelling                            Comp. Ex. 1                                                                            <5          Soft, Cracked, Swollen                                   Comp. Ex. 2                                                                            <5          Soft, Cracked, Swollen                                   Comp. Ex. 3                                                                            <5          Soft, Cracked, Swollen                                   Comp. Ex. 4                                                                             6          1/2 Pellets Soft, Slightly Swollen,                                           Other 1/2 Hard, No Cracking or                                                Swelling                                                 ______________________________________                                    

We claim:
 1. A calcined dehydrogenation catalyst comprising acalcination product of(a) at least one iron oxide; (b) a carbonate,bicarbonate, oxide or hydroxide of potassium or cesium, or a mixture ofat least two thereof; (c) an oxide, carbonate, nitrate, or hydroxide ofcerium, or a mixture of at least two thereof; (d) an hydraulic cement;(e) an hydroxide, carbonate, bicarbonate, acetate, oxalate, nitrate, orsulfate of sodium, in an amount sufficient to provide from about 0.8 toabout 10 percent sodium, calculated as sodium oxide, by weight of thecalcined catalyst; and (f) a carbonate, sulfate, or hydroxide, ofcalcium or a mixture of at least two thereof, in an amount sufficient toprovide from about 4 to 20 percent calcium, calculated as calcium oxide,by weight of the calcined catalyst.
 2. The calcined dehydrogenationcatalyst of claim 1 wherein (e) is sodium hydroxide and (f) is calciumcarbonate.
 3. The calcined dehydrogenation catalyst of claim 1containing from about 0.8 to 5 percent sodium calculated as sodium oxideand from about 3 to 15 percent calcium calculated as calcium oxide, byweight of the calcined catalyst.
 4. The calcined dehydrogenationcatalyst of claim 1 wherein (b) is a carbonate, bicarbonate, oxide orhydroxide of potassium, or a mixture of at least two thereof.
 5. Thecalcined dehydrogenation catalyst of claim 4 wherein (b) is potassiumcarbonate or a mixture of potassium carbonate and potassium oxide. 6.The calcined dehydrogenation catalyst of claim 5 wherein(c) is acarbonate of cerium; (d) is a calcium aluminate hydraulic cement; (e) issodium hydroxide, in an amount so that the catalyst contains from 0.8 to3.0 percent by weight sodium, calculated as sodium oxide; and (f) is acarbonate or sulfate of calcium or a mixture thereof, in an amount sothat the catalyst contains from 4 to 12 percent by weight calcium,calculated as calcium oxide.
 7. The calcined dehydrogenation catalyst ofclaim 1 further comprising one or more oxides of chromium, molybdenum,aluminum, vanadium, cobalt, cadmium, copper, magnesium, manganese, ornickel.
 8. A process for preparing a calcined dehydrogenation catalystcomprising:(A) preparing an extrudable mixture by admixing(a) at leastone iron oxide; (b) a carbonate, bicarbonate, oxide or hydroxide ofpotassium or cesium, or a mixture of at least two thereof; (c) an oxide,carbonate, nitrate, or hydroxide of cerium, or a mixture of at least twothereof; (d) an hydraulic cement; (e) an hydroxide, carbonate,bicarbonate, acetate, oxalate, nitrate, or sulfate of sodium, in anamount sufficient to provide from about 0.8 to about 10 percent sodium,calculated as sodium oxide, by weight of the calcined catalyst; and (f)a carbonate, sulfate, or hydroxide, of calcium or a mixture of at leasttwo thereof, in an amount sufficient to provide from about 4 to 20percent calcium, calculated as calcium oxide, by weight of the calcinedcatalyst, with sufficient water to form an extrudable mixture; (B)forming the extrudable mixture into pellets; and (C) calcining thepellets into a finished catalyst.
 9. The process of claim 8 wherein (e)is sodium hydroxide and (f) is calcium carbonate.
 10. The process ofclaim 8 containing from about 0.8 to 5 percent sodium calculated assodium oxide and from about 3 to 15 percent calcium calculated ascalcium oxide, by weight of the calcined catalyst.
 11. The process ofclaim 8 wherein (b) is a carbonate, bicarbonate, oxide or hydroxide ofpotassium, or a mixture of at least two thereof.
 12. The process ofclaim 11 wherein (b) is potassium carbonate or a mixture of potassiumcarbonate and potassium oxide.
 13. The process of claim 12 wherein(c) isa carbonate of cerium; (d) is a calcium aluminate hydraulic cement; (e)is sodium hydroxide, in an amount so that the catalyst contains from 0.8to 3.0 percent by weight sodium, calculated as sodium oxide; and (f) isa carbonate or sulfate of calcium or a mixture thereof, in an amount sothat the catalyst contains from 4 to 12 percent by weight calcium,calculated as calcium oxide.
 14. The process of claim 8 furthercomprising one or more oxides of chromium, molybdenum, aluminum,vanadium, cobalt, cadmium, copper, magnesium, manganese, or nickel.